1. Field of the Invention
The present invention relates to a method of improving the transmission characteristics of xDSL (x Digital Subscriber Line) system that effects high-speed data communication over existing telephone lines implemented by copper wires and connecting an office and subscribers and a method of measuring the transmission characteristics and more particularly to a measure against cross-talk noise on xDSL channels.
2. Description of the Background Art
The characteristics of xDSL channels deteriorate due to cross-talk ascribable to high-speed transmission relying on metallic cables, as has been pointed out in the past. A measure against cross-talk noise is particularly essential with an xDSL system in which cross talk ascribable to an ISDN (Integrated Services Digital Network) occurs in an xDSL transmission frequency band.
Generally, frequency bands assigned to xDSL systems partly overlap each other and frequency bands assigned to other high-speed communication systems including a TCM-ISDN (Time Compression Multiplexing-ISDN) system. The characteristics of each xDSL system therefore deteriorates due to cross-talk noise introduced from the other high-speed communication systems. In light of this, it has been proposed to confirm the characteristics of an xDSL channel via confirming means that faces a terminated device at the subscriber side of a subscriber line, which accommodates an xDSL subscriber, thereby selecting a channel suffering from a minimum of cross talk. Japanese Patent Laid-Open Publication Nos. 10-303872 and 2000-32096, for example, each propose to reduce cross talk by causing an xDSL channel to operate in synchronism with a TCM-ISDN transmission/receipt window.
The confirming means, however, needs a considerable period of time for selecting a channel suffering from a minimum of cross talk. The synchronous operation scheme taught in the above Laid-Open Publications applies TCM to existing xDSL systems directed toward full-duplex data transmission. This kind of scheme is therefore not practicable without changing the xDSL transmission system itself.